A video graphics system is described in British Patent No. 2,089,625 and corresponding U.S. Pat. No. 4,514,818, the teachings of which are incorporated herein by reference. This system includes a touch tablet and stylus combination for generating position signals designating the position of the stylus relative to the touch tablet. The user or operative paints or draws (hereinafter referred to simply as "paints") by selecting a colour and a so called brush distribution from a range of predefined colours and distribution functions. Movement of the stylus relative to the touch tablet causes the generation of a series of position signals.
In equipment according to our British patent no. 2,089,625, as currently manufactured and sold by the Assignee of this application, additional means are provided which allow the system to be configured to perform picture composition in addition to painting. Some configurations which allow composition are described in British patent no. 2,113,950 and in corresponding U.S. Pat. No. 4,602,286 the teachings of which are also incorporated herein by reference. In such configuration, a store is provided having the capacity to store two independent pictures, termed herein as the foreground image and the background image respectively, and a further image which is termed the control image or stencil. The control image, like the foreground and background images, can be created by the artist, using the system in a painting mode. The control image usually comprises areas of pixels having a maximum value which represents opacity and a minimum value which represents transparency and is created so that in effect it masks a selected part of the foreground image while exposing the remainder. The control image is defined by eight bits and during creation can be displayed as a single colour superimposed on the foreground picture. The pixels of the foreground image and the control image are read in parallel from the respective parts of the store and applied to a linear interpolating circuit, which is rendered operational during an image preview mode and during an image composition mode. The pixels in the background image are also read to the interpolating circuit concomitantly with the reading of the foreground image pixels and the control image pixels.
A manipulator under stylus control or the control of other user operable means, changes the order of reading the foreground pixels and control pixels relative to the background pixels so as to effect a spatial transformation of the foreground and stencil relative to the background. Whilst the system is in preview mode, the artist can manipulate the stylus to effect a series of desired transformations such as zoom, pan, scroll, rotate, and change perspective. During a frame period the foreground image pixels in transformed order and the background image pixels are applied to an image input of the interpolating circuit whilst the control image is applied to a control input thereof to act as a pixel-by-pixel interpolating coefficient. The output of the interpolating circuit is applied to the picture monitor and displayed, but without affecting any of the pictures as stored. The interpolation is of the form KB+(1-K)F, where K is an interpolation constant defined by the control image, B is the background picture data and F is the foreground picture data. The interpolation has the effect of causing the display to represent the foreground image where the control image data K=0, and to represent the background image where the control image data K=1, and to represent a combination foreground and background where K has a value between 0 and 1, thereby simulating the pasting or overlaying of the part in the foreground image on the background. Using the stylus to vary the spatial transformation, the artist can try various positions of the insert onto the background image from the foreground image until he perceives a desired transformation. Thereupon the artist, by producing a predetermined "stick" command, switches the system to the composition mode. The foreground image pixels and background image pixels are then read to the interpolating circuit and combined by interpolation under control of the control image pixels, the foreground image and control image pixels being read in the transformation order prescribed in the last try during the preview mode. The thus combined pixels are then written back to the store replacing the pixels previously stored therein.
The features and facilities provided by the above discussed systems can be applied to television post production in the form of an editing system in which a first or foreground video clip is inserted into a second background video clip. Such editing systems nowadays have the ability to capture and retouch a frame of live video, thereby enabling dynamic graphics to be produced.
Hitherto known editing systems vary from manufacturer to manufacturer but generally comprise the features shown in FIG. 1 of the accompanying drawings. As shown in FIG. 1 of the accompanying drawings a known editing system 1 comprises two sources 2 and 3, which may for example be video tape recorders (VTRs), whose outputs are connected to a vision mixer 4 which together with the two sources 2 and 3 are controlled by an edit controller 5. The system 1 further comprises a graphics unit 6 such as the abovementioned system disclosed in the British patent 2,089,625 and an effects unit 7 by which spatial transformations, such as those disclosed in the abovementioned British Patent 2,113,950, may be applied to video from one or both of the sources 2 and 3.
The system 1 can be used to apply graphics and effects to video from one of the sources, for example the source 2. In this mode of operation, a video clip is output a frame at a time from the source 2 to the effects/graphics unit 6, 7 where desired modifications are effected by the user to the frame. Once the frame has been modified it is transferred back to the source 2 and the next frame in the clip is output to the unit 6, 7. During modification the frame is output for display on a monitor 8 so that the user can see the result of his modifications as they are made to the frame.
The system 1 can also be used to combine video clips from the two sources 2 and 3, which clips may be modified as described hereinabove prior to being combined. One way in which the clips may be combined is simply to join or splice one clip to the end of the other or to splice one clip or a portion of that clip into a location between frames of the other clip. In this mode of operation the edit controller 5 is made responsive to an edit decision list which identifies the location of each clip in the two sources by for example its start frame and end frame, the number of frames in the clip and where appropriate the frames between which a splice is to be made. A more sophisticated operation which may also be performed by the system 1 is that of combining two clips by dissolving between the two clips. In a dissolving operation the edit decision list includes data identifying the number of frames in a dissolve. In response thereto the edit controller 5 controls the mixer 4 to combine corresponding frames in the two clips, for example in a weighted summing of the frame data. During this mode of operation frames from the source 2 may be modified by the effects unit 7 before being passed to the vision mixer 4.
In all of the above discussed modes of operation the resulting video output from the mixer 4 may simply be displayed on the monitor 8 or it may be stored to enable further work to be carried out subsequently. In the case where it is stored for subsequent work a further VTR (not shown) will be required to record the video output. A further VTR is necessary since both of the source VTRs 2, 3 will be engaged in playing out the two video clips and therefore will not be available for storing new frames as they are created.
The abovedescribed system 1 therefore may include a third VTR (not shown) which is used to record the combined clip output from the mixer 4. Such a system is known as a three machine edit suite, since it comprises three VTRs. A further VTR may be added to the system as a third source (not shown) to form a so-called four machine edit suite. During editing the third source is sued to supply a moving black and white key, known as a "matte reel". The key is related to the image in a clip supplied by one of the sources 2, 3 and is used in the mixer 4 to key that clip over the video supplied by the other of the sources 2, 3. Frames from the matte reel may be passed together with corresponding frames from the clip of the source to which it is related to the effects unit for combination with respective frames from the other source by the vision mixer 4.
One drawback with the abovedescribed three and four machine edit suites is that often more than one operation is required to produce a desired result and this restricts artistic use of the system. Furthermore, it is impossible to create graphics dynamically, ie as the video clips are run and this prevents subtle integration of the graphics with the video resulting in a discernable difference between the live video and the off-line graphics. As a result, it is difficult to create first time around exactly what was intended and the creation process is therefore one of repeatedly working and reworking a piece until the desired result is achieved.
Another drawback is related to the use of VTRs to supply and record video clips. VTRs are sequential access devices and cannot simultaneously playback and record different video clips. This means that a separate VTR is required as a source for each video clip to be worked on and at least one further VTR is required to record the video output from the mixer.
The proprietors of the abovementioned British and US patents manufacture and sell an editing system under the trade mark "HARRY". In the HARRY system frames forming a video clip are digitally encoded and stored in disk stores. As is disclosed in U.S. Pat. No. 4,688,106, the teachings of which are incorporated herein by reference, the video clips are read out of the disc stores in parallel. Whilst this arrangement provides greater flexibility by effectively allowing random access to the frames that constitute a clip, it nevertheless requires separate disc stores for each of the video clips.